A conventional solid fuel fired steam/electric power plant includes the following process steps:
(1) Providing a fuel yard where the fuel is received, stored and reclaimed.
(2) Preparing the fuel which may include drying, comminution, tramp removal, and screening.
(3) Further drying and thermally decomposing the fuel on a grate. The resulting combustible char, gases, and vaporized liquids are ignited and burned in the presence of excess air in a combustion section.
(4) Delivering heat from the combustion section to the boiler tubes for the generation of superheated steam from pressurized water.
(5) Moving the pressurized, superheated steam into a steam turbine where pressure and thermal energy are converted to mechanical energy by means of a Rankine cycle.
(6) Converting the mechanical energy into electrical energy via an electrical generator and transmission of that energy to the customer.
(7) Condensing the steam back to liquid water and purification of the water before returning it to the boiler system.
(8) Cleaning the gaseous products of combustion leaving the boiler system to remove particulate and gaseous contaminants prior to their discharge to atmosphere.
An activated carbon manufacturing facility includes the following process steps:
(1) Providing a feedstock yard where a solid carbonaceous material is received, stored and reclaimed.
(2) Preparing the feedstock which may include drying, comminution, tramp removal, and screening.
(3) Further drying and decomposing pyrolytically the feedstock by heating in the absence of oxygen to the temperature at which the material dries and thermally decomposes to produce combustible gases and vapors and a solid charcoal material (char) high in fixed carbon.
(4) Activating the char, where steam or other mild oxidizing agent is reacted with the char to increase its porosity. Pyrolysis and activation can take place in the same vessel. Combustion of the pyrolysis and activation off-gases provide some or all of the heat for pyrolysis and activation. The term “off-gases” as used herein means gases and vaporized liquids which are given off by a solid due to heat or reaction.
(5) Generating steam to react with the char.
There are commonalities between the biomass power production and activated carbon production technologies and there would appear to the economic benefits in sharing operations. For example, Srinivasachar in U.S. Pat. No. 7,981,835 teaches that feedstock handling and preparation can be shared by both technologies. Similarly, excess gases from the activated carbon production process can be returned for combustion in the boiler system and to be cleaned up in the existing power plant. The actual scope of process integration taught by Srinivasachar is limited.
Despite the commonalities, the prior art does not incorporate the respective two processes, namely the combination of the Rankine cycle steam generation with activated carbon production, within a single common housing and utilizing combustion to heat both (1) water to produce steam as well as (2) the activated carbon precursors. In addition, the prior art does not synergistically utilize the products of the pyrolysis process to augment the continuing overall process operation.
Both Majmudar U.S. Pat. No. 8,999,885 and Srinivasachar U.S. Pat. No. 7,981,835 teach the production of steam for power generation and also the production of activated carbon, however, in both patents, more emphasis is given to power generation. Majmudar suggests that the char separated from a partial oxidation or gasification reactor can be heated in a screw conveyor to a temperature between 400 and 1100° C. to activate the charcoal. However, it is known in the art that heat alone will not activate charcoal, particularly charcoal which has been exposed already to temperatures of 800° C. or higher in a biomass gasifier. In another embodiment taught in Majmudar, the char, preheated to similar temperatures in a screw conveyor, is then exposed to steam, the maximum temperature of which is unlikely to exceed 650° C. after being heated by gas turbine exhaust. Char activation is endothermic and the reaction mix would quickly cool before significant activation is accomplished. Even if it is assumed that the charcoal could be activated, the charcoal from a gasification reactor is very high in ash, typically 20 to 50% and even higher from some gasifiers. Under that scenario, the activated carbon would be of low quality. While much of the ash can be removed by an acid wash, as is suggested in Majmudar U.S. Pat. No. 8,999,885, there remains the pollution associated with such a process. That, combined with the low charcoal yield from gasification, typically less than 5% and in some gasifiers less than 1%, limits the economic attractiveness of the overall process.
Typically, one ton of biomass produces about one megawatt-hour of electric energy, which has a value of $30 to $40 when sold as baseline power. On the other hand, one ton of biomass can produce 0.1 to 0.15 tons of activated carbon which is worth about $2000 per ton, for an equivalent total value of $200 to $300 per ton of biomass. Accordingly, to ensure maximum viability, process integration must favor the production of activated carbon over power generation, which is not the case in either of the Majmudar U.S. Pat. No. 8,999,885 or Srinivasachar U.S. Pat. No. 7,981,835 patents. The present process maximizes the activated carbon yield while completely integrating char production and activation inside a single housing to optimize production for maximum economic benefit.
From the above, it is therefore seen that there exists a need in the art to overcome the deficiencies and limitations described herein and above.